Alkylation and transalkylation processes using catalysts are often subject to catalyst regeneration and replacement requirements resulting from poisoning of the catalyst by one or more impurities contained in the hydrocarbon feedstock. In many cases, catalyst developments, e.g. to reduce coke-forming and other by-product reactions, have progressed to the stage where poisoning by feedstock impurities is the primary reason that catalyst performance deteriorates which forces the catalyst to be replaced or regenerated. Aromatics alkylation processes employing molecular sieve catalysts can be conducted in either the vapor phase or the liquid phase. However, in view of the improved selectivity and decreased capital and operating costs associated with liquid phase operation, most commercial alkylation processes now operate under at least partial liquid phase conditions. Unfortunately, one disadvantage of operating under liquid phase conditions is that the molecular sieve catalysts tend to be more sensitive to the presence of impurities in the feedstocks, particularly polar compounds such as nitrogen compounds. Such impurities reduce the acid activity of the catalyst and hence decrease the cycle time between required regenerations of the catalyst. Various processes have been developed for removal of such impurities prior to contact with the catalyst. In the following prior arts there are description of said impurities as well as processes to remove these impurities.
U.S. Pat. No. 6,002,057 describes a process for the alkylation of an aromatic hydrocarbon contained in a hydrocarbon stream comprising:
(a) separating essentially all aromatics other than said aromatic hydrocarbon from said hydrocarbon stream, thereby forming an aromatic-rich stream;
(b) treating said aromatic-rich stream of (a) by converting essentially all olefinic compounds contained therein by hydrogenation;
(c) contacting the thus treated aromatic-rich stream from (b) with an olefin-containing stream comprising at least one olefin selected from the group consisting of ethylene, propylene, and butylene, wherein the molar ratio of the olefin(s) to said aromatic hydrocarbon is not less than 1 in the presence of a catalyst comprising zeolite beta, under alkylation conditions, whereby mono and polyalkylated aromatics are formed; and(d) separating said mono and polyalkylated aromatics formed in (c) from the remaining hydrocarbons.
At col 6 lines 48-53 is mentioned that the olefins may be present in admixture with hydrogen, methane, C2 to C4 paraffins, but it is usually preferable to remove dienes, acetylenes, sulfur compounds or basic nitrogen compounds (NH3 or amines) which may be present in the olefin feedstock stream, to prevent rapid catalyst deactivation.
U.S. Pat. No. 6,617,482 describes a process for removing polar compounds from an aromatic feedstock which contains polar compounds and which is then used in an alkylation process. The process comprises contacting the feedstock in an adsorption zone at a temperature of less than or equal to 130° C. with an adsorbent selective for the adsorption of said polar compounds and comprising a molecular sieve having pores and/or surface cavities with cross-sectional dimensions greater than 5.6 Angstroms. A treated feedstock substantially free of said polar compounds is withdrawn from the adsorption zone and fed to an alkylation zone for contact under liquid phase alkylation conditions with an alkylating agent in the presence of an alkylation catalyst. More particularly, this prior art relates to a liquid phase aromatics alkylation process which includes subjecting the aromatic feedstock to a pretreatement step for the selective removal of polar contaminants that poison aromatic alkylation catalysts. Such contaminants include nitrogen, sulfur, and oxygen containing compounds, particularly those that boil in the same ranges as benzene, toluene or xylenes.
US 2005 0143612 describes a process for the production of alkyl aromatic compounds wherein aromatic compounds which may be treated for removal of deleterious substances are reacted with olefin compounds, which may also be treated for contaminant removal, in the presence of acidic zeolite catalyst(s) to produce the desired alkyl aromatic compound(s). The aromatic and preferably also the olefin feeds are treated substantially to remove contaminants, particularly the nitrogen compounds contained therein, before they are brought together for reaction in the presence of the zeolite catalyst(s). The feed pretreatment for removal of nitrogen compounds significantly improves the run length and life of the acidic zeolite catalyst(s). A specific object of this prior art is to provide methods and apparatus for treating an olefin or aromatic feedstock for removal of organic or inorganic nitrogen compounds in preparation for a catalytic alkylation or transalkylation process. In accordance with said prior art, it has been found that nitrogen-containing impurities in one or both feedstocks may neutralize the acidic active sites on the acidic zeolite catalyst and thereby reduce catalyst activity and its ability to effect the desired reaction. Long-term accumulation of these nitrogen-containing impurities on the catalyst gradually reduces catalyst activity to the point where plant performance becomes unacceptable, requiring that the plant be shutdown to reactivate, regenerate, or replace the catalyst.
U.S. Pat. No. 7,199,275 relates to a process for alkylation of an aromatic hydrocarbon stream comprising impurities in which said impurities are removed in a pretreatment system having a first stage, a second stage located downstream of said first stage and a cycle length, said process comprising the steps of:
(a) contacting the aromatic hydrocarbon stream with a first molecular sieve which is 13× in said first stage of said pretreatment system, to remove at least a portion of said impurities, to produce a partially treated aromatic hydrocarbon stream;
(b) contacting said partially treated aromatic hydrocarbon stream with a second molecular sieve which is 4 A in a second stage of said pretreatment system to remove substantially all of the remaining portion of said impurities, and to produce a fully treated aromatic hydrocarbon stream; and(c) contacting said fully treated aromatic hydrocarbon stream with an alkylating agent in the presence of an alkylation catalyst and under alkylation conditions, to produce an alkylated aromatic hydrocarbon stream; and wherein said cycle length of said pretreatment system is greater than said cycle length using said first stage of said pretreatment system alone or said second stage of said pretreatment system alone.
In a preferred embodiment the process further comprises the step of contacting said alkylating agent of step (c) with a third molecular sieve, to produce a treated alkylating agent, and contacting said treated alkylating agent with said treated aromatic hydrocarbon stream of step (b), to produce said alkylated aromatic hydrocarbon stream of step (c).
However the above prior arts are silent on how to check these impurities in an industrial process. It has now been discovered that by dry colorimetry it was possible to get in less than 5 minutes the amount of the nitrogen containing compounds impurities in the aromatic substrate or in the alkylating agent. A sample of the feedstock to be analyzed is vaporized, unless it is available under gasous phase, and is sent through a porous substrate carrying a reagent which changes color in proportion with the impurities. An electronic device converts the color to the amount of impurities. To improve the response in time, many such apparatus can be used in parallel and a measurement is launched e.g. every minute or every ten seconds. Should the impurities are found to be higher than a requested level the reactor containing the catalyst can be temporarily by-passed.